1. Field of the Invention
The present invention relates to a method for removing water vapor from a water vapor-containing gas. More particularly, the present invention relates to a method for selectively removing water vapor from various water-containing mixed gases, for example, air, natural gas, mixed gases accompanying crude oil production, and fermentation gases, by means of a gas separating membrane which has a high separating selectivity for water vapor and may be in the form of a hollow-filament, a spiral film or a flat film, and preferably comprises an aromatic imide polymer.
2. Description of the Related Art
It is known that water vapor can be removed from water vapor-containing mixed hydrocarbon gas, for example, natural gas by a glycol absorption method or molecular sieve adsorption method.
However, the above-mentioned known methods are disadvantageous in that they necessitate the use of large equipment, with the inevitable high costs and need for a large installation space, and thus, are not satisfactory for undersea crude gas production in which the operation space is very restricted. Additional problems are those of the complicated operation, operational safety and maintenance of such equipment.
In place of the above-mentioned adsorption methods several new methods, which are carried out by a small size, light weight apparatus which is easily maintained and operated under enhanced safety conditions, have been developed. In those new methods, water vapor is removed from various water vapor-containing mixed gases by means of a gas separating module containing at least one gas separating membrane therein.
The water vapor removing methods utilizing the gas separating membrane include, for example, the following methods.
(a) Japanese Unexamined Patent Publication No. 54-152679 discloses a water vapor-removing method wherein a water vapor-containing mixed gas is fed at a feed side of a gas separating module, under a very high pressure at the feed side or under a reduced pressure at the delivery side of the gas separating module, so that there is a large difference in pressure between the feed side and the delivery side of the gas separating module.
(b) Japanese Unexamined Patent Publication No. 59-193835 discloses a water vapor-removing method for various water vapor-containing mixed gases containing, as a major component, a gas having a high permeability, for example, natural gas containing, as a major component, methane gas, in which method a gas separating membrane has a ratio (PH.sub.2 O/PCH.sub.4) of a water vapor-permeating rate (PH.sub.2 O) to a methane gas-permeating rate (PCH.sub.4) of 200 to 400.
(c) Japanese Unexamined Patent Publication (Kokai) No. 50-2674 discloses a water vapor-removing method in which a large amount of a purge gas is fed in a delivery side of a gas separating membrane so as to decrease the partial pressure of water vapor permeated through the gas separating membrane and to promote the permeation of the water vapor.
In the known method (a), the resultant gas fraction permeated through the gas separating membrane sometimes has a high content of water vapor at the delivery side of the gas separating module, especially in an outlet region for the permeated gas fraction. This phenomenon causes the water vapor-removing efficiency to be decreased. Accordingly, the known method (a) is not satisfactory for practical use.
In the known method (b), it is difficult to obtain a satisfactory gas separating membrane for this method. Also, the method (b) is disadvantageous in that a large amount of the valuable component of the water vapor-containing mixed gas to be recovered is lost, and therefore, the recovery efficiency of the valuable component is poor.
The known method (c) is disadvantageous in that a very large amount of the purge gas is consumed.